Objectives: (1) To elucidate the nature of the change that conditions explanted tumors and normal endocrine tissues to grow in allogeneic hosts; (2) to account for the specific decrease in immunogenicity that takes place in allografted tumors (conditioned in vitro or in vivo) that allows them to become serially allotransplantable in specific strains and (3) to explore the reactions of the allograft hosts which contribute to the allograft perpetuation. Approaches: (1) Characterization of soluble membrane composition of immunologically modified tumors and study of the in vivo activity of the solubilized antigens (2) Search of possible changes in stromal versus parenchymal cell composition in cultured explants and allotransplantable tumors derived from them (3) Attempts to reverse modifications reducing immunogenicity of tumors.